This invention relates generally to methods and apparatus for use in connection with lighter-aboard-ship (LASH) vessels of the floater type and, more particularly, to a method and apparatus for facilitating the entry and exit of lighters into and from such lighter-aboard-ship vessels and, additionally, to prevent the entry of foreign objects onto the docking deck due to the inflow of water into the docking space.
As is known, a lighter is a boat which is used for loading or unloading vessels at locations remote from wharves or the like. A lighter-aboard-ship (LASH) vessel of the floater type is characterized by the incorporation therein of a docking space which can be selectively filled with water so as to enable a lighter to dock therewithin and subsequently emptied of water to allow the loading or unloading of the lighter.
More particularly, when, for example, cargo is to be conveyed from a lighter to a LASH vessel, the docking space of the vessel is filled with water to an extent sufficient to permit the lighter to float within the docking space. The filling of the docking space with water is accomplished by allowing the water to flow into sinking tanks located between the hull walls until the docking deck which comprises the bed or floor of the docking space descends to at least a depth equal to the draft of the lighter in the surrounding waters. Water is introduced into the docking space through valves which are conventionally located in the docking space bed.
The lighter or lighters can then be floated into the basin defining the docking space through the usual gate or gates, the lighters being urged into the docking space usually with the assistance of a tug boat or the like. After the lighters are completely located within the docking space basin, the gate is closed whereupon the aforementioned tanks located in the hull of the vessel are evacuated and the docking space emptied of the water contained therein. In this manner the LASH vessel rises within the water to its normal operating draft and the lighters eventually come to rest on the bed of the vessels docking space.
The undocking or exit of the lighters from the docking space subsequent to unloading the cargo onto the vessel is accomplished by ballasting the vessel so that it descends to its lower draft as described above and filling the docking space basin with water in an amount sufficient to permit the lighters to float within the docking space. The lighters can then float through the docking space gate into the surrounding water again, preferably, with the assistance of a tug or the like.
The above-described conventional procedure for docking and undocking lighters in LASH vessels has a serious problem inherent therein. More particularly, when the lighter floats into the docking space as described above through the gate thereof or, conversely, when the lighter floats out of the docking space through its gate, a volume of water is displaced from and into the docking space in a direction opposite to the direction of travel of the lighter in the region of the gap defined between the lighter and the docking space gate at a velocity which is many times greater than the velocity of the lighter. Such an opposite high velocity displacement of water results in a high resistance to the travel of the lighter as it moves through the gate thereby impeding the docking and undocking procedures. Additionally, in the case where the lighter exits from the docking space, i.e. in the case of undocking, it is not uncommon for foreign objects such as ice floes to be carried into the docking space with the inflowing water creating serious problems of debris and the like.